1. Technical Field
This invention relates to liquid level sensors, and more particularly to a capacitive liquid level sensing apparatus incorporating phase detecting circuitry for detecting and providing an output of a fluid level within a fluid reservoir.
2. Discussion
The emergence of composite fuels and the need for much better accuracy in the determination of fluid levels within fluid reservoirs has resulted in new requirements for fuel gauges of various vehicles, and also the drafting of new requirements for fuel gauges by the automotive industry. At present, previously unmeasured fluid levels such as those associated with transmission fluid, engine oil and radiator coolant are now being considered for continuous level sensing. The available methods for measuring variable fluid type levels heretofore have not provided sufficient accuracy of the sensed liquid level, have been ill-suited to the hostile environments of various fluids or are very costly to manufacture.
A variety of technologies and techniques have been used thus far in fluid level sensor designs. Some of the designs meet many of auto industry requirements but, to date, a completely satisfactory fluid level sensor design has yet to be produced which meets all present day auto industry requirements.
The largest category of fluid level sensor designs involve an approach which makes use of a capacitive sensing probe disposed in the fluid to be monitored. The capacitive probe is typically positioned vertically within a reservoir containing the fluid and along the axis of measurement such that the amount of area of the capacitive element of the probe which is covered by the fluid in the reservoir ultimately provides an indication of the level of the fluid.
The capacitive probe type Sensor typically utilizes some form of capacitance measurement to determine the fluid level. With these systems the dielectric constant of the fluid relative to that of air is the key to the accuracy of the measurement. The range of the probe capacitance is highly dependent on the dielectric constant of the fluid. The relative dielectric constant of air is near unity and changes negligibly under varying ambient conditions. The relative dielectric constant of the fluid, however, can change dramatically. The conductance of the various fluids typically monitored can vary dramatically, which in turn can significantly impact the dielectric constant since it is comprised of both a real and imaginary component.
Some previously designed systems incorporating capacitive probe technology have involved going to great lengths to fully characterize the dielectric constant to effect an accurate measurement. This determination is typically accomplished by some form of in-situ measurement of the dielectric constant. Other approaches attempt to avoid the effects of the varying dielectric constant by attempting to remove the variation from the measurement. One such method to avert the dielectric constant variation has been to segment one or both of the capacitor elements of the capacitive probe into an array of smaller elements whose axis aligns with that of the measured fluid level. Provided that a sufficient number of independent capacitive segments is provided, the most simple form of this measurement becomes the determination of the absence or presence of the dielectric material (i.e., the fluid) in this instance.
Recently released requirements, typical of the auto industry at large, for a fuel level sensor are listed below to provide an idea of the stringency of present day operational parameters which a fuel level sensor must meet:
______________________________________ A. Temperature Range - (-)40.degree. C. to 150.degree. C. B. Life - 20 Years C. Response Time - 15 milliseconds D. Accuracy 0.5 gallons minimum 0.1 gallons preferred E. EMI/RFI - Must be operational in close proximity to fuel pump F. Fuel Tolerance - Sensor must be capable of meeting accuracy requirements for the following fuel types: TF1 TF2 UNLEADED GASOLINES 100% INDOLENE HO-III PEROXIDE FUEL MIX METHANOL FUEL MIX CORROSIVE GASOHOL Additionally, the sensor must be capable of limited exposure to 2 RVP Fuel as well as not being adversely affected by exposure to legal and commercial fuels in the Asian, Mideast and European markets. G. Underbody Contaminants - The sensor must withstand prolonged exposure to the following list of potential underbody contaminants: Engine Oil Transmission Fluid Power Steering Coolant/Antifreeze Fluid Windshield Wash Fluid Brake Fluid Wheel Bearing Lube Transaxle/Differen- A/C Refrigerant tial Lube Acid Rain Water Waxes, Paint Sealants Snow, Ice Tire Cleaners Car Wash Chemi- Carpet Cleaners cals Steam Cleaning Engine Cleaning Soft Drinks, Coffee, Etc. H. Space/Size Requirements - The sensor shall be contained within the following form factors: 15 .times. 4 .times. 175 MM 15 .times. 4 .times. 400 MM 10 .times. 6 .times. 175 MM 10 .times. 6 .times. 400 MM I. Electrical Requirements - Operational Voltage: 10.5 to 16.5 volts Output voltage: 0 to 4.8 volts linearly related to measured level. J. Mechanical Requirements - Sensor must survive a three feet vertical drop and still meet the electrical requirements. ______________________________________
Coupled with the above requirements is a demand for a low manufacturing cost. This places an additional constraint on the design of the liquid level sensing system. In summary, the fluid level sensing system must accurately measure a variety of materials in a hostile environment and be capable of manufacture at a relatively low cost.
Accordingly, it is a principal object of the present invention to provide a liquid level sensing apparatus incorporating a capacitive probe in which the accuracy of the apparatus is not affected by changes in the dielectric constant due to the temperature of the fluid, the composition of the fluid or other factors which cause a variation of the dielectric constant.
It is another object of the present invention to provide a liquid level sensing apparatus which is capable of at least meeting typical auto industry requirements for a fuel level sensor, while still being capable of manufacture at a relatively low cost.
It is yet another object of the present invention to provide a fluid level sensing apparatus which is capable of determining differences in the composition of the fluid being sensed without adding significant complexity to the determination of the fluid level measurement.
It is still another object of the present invention to provide a fluid level sensing apparatus which incorporates a capacitive sensing probe which is relatively simple in construction and well adapted to withstand the hostile environment of a variety of fluids typically used in motor vehicle applications for lubrication and/or cooling purposes.
It is still another object of the present invention to provide a liquid level sensing apparatus whose operation is generally not effected by bumps, jolts, or other like movements typically experienced with moving motor vehicles, which movements could adversely effect the accuracy and/or operability of prior art liquid level sensing systems.
It is yet another object of the preset invention to provide a liquid level sensing apparatus which incorporates phase detecting circuitry for accurately detecting a fluid level within a fluid reservoir, the accuracy of which is not adversely affected by changes in the composition of the fluid, its temperature or any other environmental factor acting on the fluid.
It is still another object of the present invention to provide a fluid level sensing apparatus which quickly, in real-time, can respond to very rapid changes in the level of a fluid and provide an accurate indication in real-time of the level of the fluid within a given reservoir.